Ceramic multilayer circuit boards have been used for many years for circuits for electrical apparatus. They are made by casting glass and/or ceramic powders, such as low firing temperature devitrifying glasses having a firing temperature below about 1000.degree. C., together with an organic binder, to form a green tape. A conductive metal circuit is formed on the green tape by screen printing. These green tapes may be stacked together to form a multilayer circuit stack. The circuits on each green tape layer are connected electrically by punching via holes in the green tape, which are then filled with a conductive material. The green tapes are then aligned and pressed together, or laminated, and fired to remove the organic materials and to sinter the glass.
More recently, the green tape layers have been aligned on a conductive metal support substrate, to enhance the electrical, mechanical and thermal properties of multilayer glass/ceramic circuit boards. The multilayer ceramic green tapes are adhered to the support substrate by means of a bonding glass. Proper choice of bonding glasses can both adhere the green tape layers to the conductive support, and can suppress shrinkage of the green tape during firing in the x and y lateral dimensions. The resultant green tape stack, which shrinks up to about 20% by volume during firing, shrinks only in the z direction, that is, the vertical direction with respect to the support substrate. This eliminates problems of alignment between the green tape stack and the support substrate, and improves via hole tolerances of all substrate features.
When it is desired to pattern the green tape stacks so as to form cavities in the green tape in which, after firing, devices such as silicon chips may be placed, the presence of the support substrate has reduced the shrinkage and deformation of such cavities during firing. A prior art process has been described that shows process steps for making the cavities and for limiting their deformation during firing, as shown in FIGS. 1A, 1B, 1C and 1D.
FIG. 1A illustrates a green tape layer stack 22 having a cavity 24 formed therein. The green tape layer stack 22 is supported by a substrate support 26 and has a top layer 28 of a non-sintering ceramic material. This ceramic material can be a high sintering temperature material such as alumina.
A machined insert or cavity plug 29, such as of ceramic of the size of the cavity 24, is placed in the cavity 24 to maintain the size of the cavity during lamination and firing as shown in FIG. 1B.
After firing the laminate stack and removing the insert 29, as shown in FIG. 1C, the sintered layers 32 have shrunk only in the thickness dimension, and the top layer 28 remains intact, still slightly overhanging the fired stack 32. The layer 28 now comprises a powdered ceramic. The cavity 24 has improved dimensional stability, and has about the same dimensions as the original cavity.
FIG. 1D shows the fired green tape stack after removal of the top ceramic layer and the machined insert. The fired stack 32 having a cavity 24 therein is adhered to the substrate 26.
However, although the shrinkage in the x and y directions can be greatly reduced in accordance with the above process, there is still some shrinkage during firing in the lateral dimensions, up to about 3%, and it is difficult to retain both the shape and the size of the cavities formed in the green tape throughout the firing step. Deformation occurs because, during firing, the cavity walls flow inwardly, so that the fired cavity is smaller than the cavity originally punched in the green tape. When a cavity plug, made of a high firing temperature ceramic is placed in the cavity prior to firing in a further attempt to prevent shrinkage, the shrinkage that still occurs in the z direction also contributes to the deformation of cavity walls, and it is difficult to maintain the flatness of the cavity walls and floors because of the flow of resin and glass during the firing step. Ceramic plugs or inserts, when used, have the disadvantages that they must be separately formed, and must be separately removed from the cavities after firing. These requirements add to the number of steps required to make the patterned multilayer printed circuit board, and thus add to the costs of such a process. However, the fired shrinkage is only marginally improved, and some shrinkage still occurs in lateral directions.
Thus a method of improving the dimensional control and deformation control during lamination and firing of multilayer, patterned green tape stacks would be highly desirable.